1. Field of the Invention
The present invention relates to a wafer transfer device and more particularly, a device for transferring wafers between wafer boat and cassette.
2. Description of the Related Art
A wafer housing case which is called carrier or cassette is usually used to transfer semiconductor wafers and others in the course of manufacturing semiconductor devices.
The wafer cassette is made of resin, for example, light in weight and low in cost, and it is designed to house plural or 25 sheets of wafers therein.
When a plurality of wafers are to be processed as a batch by the heat process system, they cannot be heat-processed under the condition that they are kept housed in the resin-made cassette. It is therefore usually needed that they are heat-processed after they are transferred into the heat processing wafer boat made of quartz, for example, chemically stable and excellent in heat resistance.
As shown in FIGS. 1 and 2, a plurality wafer-mounted levels are vertically arranged in a wafer boat 10 so as to process a plurality of wafers as a batch. A ring-shaped support plate 11 having a recessed support face 11a thereon (see FIG. 2) has been used these days as the wafer-mounted level, because the circumferential portion of the wafer can be surrounded by the ring-shaped support plate 11. When the wafer is supported in this manner, its temperature can be uniformly lowered and raised so as to form a film of uniform thickness on the wafer.
When the wafer is transferred between the ring-shaped support plate and a transfer fork, however, the up- and down-movement of the fork is disturbed by the ring-shaped support plate. This makes it necessary to move the wafer up and down independently of the fork. To meet this need, an inventor of the present invention has provided in U.S. patent application 07/572,005 (now U.S. Pat. No. 5,162,047 issued Nov. 10, 1992) double fork structure in which two wafer transferring and pushing-up forks are used.
When this double fork structure is employed, however, intervals between the wafer-mounted levels or ring-shaped support plates 11 must be made large to insert it.